This invention relates to certain hyperbranched organometallic polymers which are useful as precursors to certain ceramic materials, processes for the preparation of such polymers and the preparation of ceramic materials from such polymers, ceramic materials and their use as ferromagnetic materials and electrically conductive materials.
According to a first aspect of the present invention there is provided a process for the preparation of a hyperbranched organometallic polymer which comprises reacting dilithioferrocene or a complex thereof with a compound of the general formula RSiX3 in which R represents a hydrogen atom or an optionally substituted alkyl, alkenyl or aromatic group and each X independently represents a halogen atom, optionally in the presence of an anionic initiator.
Preferably, the compound of general formula RSiX3 is added as a solution in an ether solvent such as tetrahydrofuran. The reaction is conveniently carried out at a temperature from xe2x88x9280xc2x0 C. to room temperature (20 to 30xc2x0 C.).
In this specification, any alkyl group, unless otherwise specified, may be linear or branched and may contain up to 24, preferably up to 20, and especially up to 18, carbon atoms. Preferred alkyl groups are n-alkyl groups, that is, linear alkyl groups, with methyl, octyl, dodecyl, hexadecyl and ociadecyl groups being especially preferred.
Any alkenyl group, unless otherwise specified, may be linear or branched and may contain up to 12, preferably up to 6, and especially up to 4, carbon atoms. Linear alkenyl groups are preferred and ethenyl (vinyl), propenyl and butenyl groups are especially preferred. Ethenyl groups are particularly preferred.
An aromatic group may be any aryl or heteroaryl group, with aryl groups being particularly preferred. An aryl group may be any monocyclic or polycyclic aromatic hydrocarbon group and may contain from 6 to 14, especially 6 to 10, carbon atoms. Preferred aryl groups include phenyl, naphthyl, anthryl and phenanthryl groups, especially a phenyl or naphthyl, and particularly a phenyl, group. A heteroaryl group may be any aromatic monocyclic or polycyclic ring system which contains at least one heteroatom. Preferably, a heteroaryl group is a 5- to 14-membered, and especially a 5- to 10-membered, aromatic ring system containing at least one heteroatom selected from oxygen, sulphur and nitrogen atoms. Preferred heteroaryl groups include pyridyl, pyrrolyl, furyl, thienyl, indolinyl, imidazolyl, pyrimidinyl, pyrazinyl, oxazolyl, thiazolyl, purinyl, quinolinyl, quinoxalinyl, pyridazinyl, benzofuranyl, benzoxazolyl and acridinyl groups.
A halogen atom may be a fluorine, chlorine, bromine or iodine atom. Chlorine atoms are particularly preferred.
When any of the foregoing substituents are designated as being optionally substituted, the substituent groups which are optionally present may be any one or more of those customarily employed in the development of polymers and ceramic materials and/or the modification of such compounds to influence their structure/activity, stability or other property. Specific examples of such substituents include, for example, halogen atoms, nitro, cyano, hydroxyl, alkyl, haloalkyl, alkoxy, haloalkoxy, amino, alkylamino, dialkylamino, formyl, alkoxycarbonyl, carboxyl, alkanoyl, alkylthio, alkylsulphinyl, alkylsulphonyl, carbamoyl and alkylamido groups. When any of the foregoing substituents represents or contains an alkyl substituent group, this may be linear or branched and may contain up to 12, preferably up to 6, and especially up to 4, carbon atoms. A halogen atom may be a fluorine, chlorine, bromine or iodine atom and any group which contains a halo moiety, such as a haloalkyl group, may thus contain any one or more of these halogen atoms.
It is preferred that a complex of dilithioferrocene with tetramethylethylene diamine (TMEDA) is used. This complex of dilithioferrocene with TMEDA, that is, dilithioferrocene. TMEDA, may be prepared by reacting ferrocene with TMEDA in the presence of a solvent and a lithiating agent. Preferably, the solvent is a hydrocarbon solvent, such as anhydrous hexane. It is also preferred that the lithiating agent is n-butyl lithium. This reaction may be conveniently carried out at a temperature from xe2x88x928xc2x0 C. to room temperature (20 to 30xc2x0 C.).
The process steps described above may also be carried out in a one-pot reaction. According to a second aspect of the invention there is therefore provided a one-pot process for the preparation of a hyperbranched organometallic polymer which comprises reacting ferrocene with TMEDA in the presence of a solvent and a lithiating agent, optionally adding an anionic initiator, and reacting the resultant mixture with a compound of the general formula RSiX3 in which R represents a hydrogen atom or an optionally substituted alkyl, alkenyl or aromatic group and each X independently represents a halogen atom.
The lithiating agent in the above processes may be any compound which is capable of attaching lithium atoms to the ferrocene. One such lithiating agent is n-butyl lithium.
In some instances, it may be necessary or desirable to add an agent to facilitate the reaction between the dilithioferrocene or complex thereof and the compound of general formula RSiX3. For instance, it may be preferable to add an anionic initiator to get the ring moieties possibly existing in the system open. N-Butyl lithium may act as an anionic initiator in this respect. Preferably, this is added to the reaction mixture under an inert atmosphere, such as nitrogen.
As n-butyl lithium can function both as a lithiating agent and an anionic initiator, it is particularly advantageous to use this compound in the processes of the present invention.
In both processes, it is preferred that R represents an optionally substituted C1-24 alkyl or C2-12 alkenyl group. Preferably, R represents an optionally substituted C1-20, especially C1-18, alkyl group or an optionally substituted C2-6, especially C2-4, alkenyl group. Preferably, such alkyl and alkenyl groups arc linear. It is especially preferred that R represents an n-alkyl or linear alkenyl group, with methyl, n-octyl, n-dodecyl, n-hexadecyl, n-octadecyl and ethenyl (vinyl) groups being especially preferred. The optional substituents may be any of those listed previously with halogen atoms being particularly preferred.
Although each X may represent a different halogen atom, it is preferred that all three X atoms represent the same halogen atom. Preferably, each X represents a chlorine atom.
In a third aspect, the invention provides a hyperbranched organometallic polymer produced by any of the processes described above.
Such hyperbranched organometallic polymers are believed to be novel compounds. According to a fourth aspect of the invention there is therefore provided a hyperbranched organometallic polymer selected from the group consisting of poly [1,1xe2x80x2-ferrocenylenesilynes], poly [1.1xe2x80x2-ferrocenylene-(alkyl)silynes], poly [1.1xe2x80x2-ferrocenylene(alkenyl)silynes] and poly [1,1xe2x80x2-ferrocenylene(aromatic)silynes]. In these polymers, every silicon atom is surrounded on average by 3/2 (1.5) ferrocenylene moieties and one R moiety.
Preferably, the hyperbranched organometallic polymer has a general formula 
in which Fc represents a 1,1xe2x80x2-ferrocenylene group, R represents a hydrogen atom or an optionally substituted alkyl, alkenyl or aromatic group and n represents an integer greater than 1.
It is preferred that the polymer described above contains at least one moiety of the general formula 
in which Fc and R are as defined above.
The polymer may have a structure of the general formula: 
in which Fc and R are as defined above.
It is believed that such structures form when the silicon atom is in a sterically accessible environment thereby promoting a three-directional propagating reaction. This situation tends to arise when R is relatively small. Thus, polymers of the invention in which R represents a methyl or ethenyl (vinyl) group have the above structure of formula (A).
The polymer may also have a structure of the general formula: 
in which Fc and R are as defined above.
When R is a lengthy or bulky group, it is thought that such groups may encumber the silicon atom from nucleophilic attack. Also, the steric hindrance of such groups may hinder the formation of a cross-linking network. Thus, polymers of the above structure tend to form when R is a lengthy or bulky group. Polymers of the invention in which R represents a C8-24 alkyl particularly an n-C8-24 alkyl, group have the above structure of formula (B), especially polymers in which R represents an n-octyl, n-dodecyl, n-hexadecyl or n-octadecyl group.
It has also been found that the hyperbranched organometallic polymers of the invention, which tend to be three-dimensional spheres, can be pyrolysed to form ceramic materials with advantageous properties. According to a further aspect of the invention there is therefore provided a process for the preparation of a ceramic material which comprises pyrolysing a hyperbranched oragnometallic polymer as defined above under an inert gas atmosphere.
Preferably, the inert gas is nitrogen or argon. It is also preferred that the process is carried out at a high temperature, temperatures in the range from about 700xc2x0 C. to about 1200xc2x0 C. being particularly preferred.
Sintering at high temperatures under nitrogen and argon converts the polymers to ceramic materials in yields of about 48-62%. These yields are much higher than those obtained using linear counterpart polymers, as described in the prior art. The hyperbranched polymers of the invention are thus superior to the corresponding linear polymers as ceramic precursor polymers. It is thought that the three-dimensional spheres of the hyperbranched polymers serve as shielding nets to keep the atoms inside thereby enabling the high-yield production of the ceramic materials. Most of the hyperbranched polymer precursors of the invention are soluble in common solvents, melt at relatively low temperatures and can be processed into robust engineering forms such as thin films. This excellent processability is therefore a further advantage.
The process conditions affect the structure of the ceramic material end product. For instance, in general, the nanocrystals in ceramic materials prepared by pyrolysis under nitrogen are mainly xcex1-Fe2O3 whereas those in ceramics prepared under argon are mainly Fe3Si.
In a further aspect, the invention provides a ceramic material produced by a process as described above.
The invention still further provides a ceramic material containing iron, silicon and carbon which comprises a three-dimensional mesoporous network of nanoclusters.
Analysis has shoved that the ceramic materials produced by the process described above tend to have four elements on the ceramic surface, namely, iron, silicon, carbon and oxygen. The first three of these elements come from the polymer precursor. However, it is thought that the oxygen may be introduced by the moisture absorbed by the polymer samples prior to pyrolysis and/or by the post-oxidation of the mesoporous ceramics during the handling and storage processes. Ceramic materials of the invention therefore may also contain oxygen.
As mentioned above, the precise structure of such ceramic materials can be altered by varying the process conditions under which they are formed.
The iron content of ceramic materials of the invention is about 4 times higher (about 40%) than that of ceramic materials prepared from the corresponding linear precursors. Normally, ceramics prepared from polymer precursors are used as structural materials (eg. silicon carbide, silicon nitride, etc.). However, the high iron content and the nanodimensions of the iron clusters endow the ceramic materials of the invention with high electrical conductivity and supermagnetic susceptibility. The saturation magnetisability of the ceramic materials of the invention at room temperature is about 50 emu/g, which is much higher than that from the corresponding linear polymers (up to about 3 emu/g). Moreover, the hysteresis loss of the magnetic ceramics of the invention is virtually zero. Also, these materials show near-zero remanence and coercivity. Thus, the ceramics of the invention are excellent soft ferromagnetic materials.
In further aspects, the invention therefore provides the use of a ceramic material as defined above as a ferromagnetic material and the use of a ceramic material as defined above as an electrically conductive material.
These properties make the ceramic materials of the invention particularly useful for the manufacture of smart magnetic cards, magnetic thin coatings, information recording and storage media, magnetic refrigeration, color imaging, ferrofluids, cell sorting, medical diagnosis, control site- and target-specific drug delivery, nanodimensional motors and switches, reusable supported catalysts and mesporous filters.
Other features and advantages of the various aspects of the present invention will be apparent from the disclosures of Sun and Tang, Poly. Mater. Sci. Eng. 82, 105-106 (2000); Sun et al Poly. Mater. Sci. Eng. 82, 107-108 (2000): Sun, Wong and Tang, Poly. Mater. Sci. Eng. 82, 109-110 (2000); and Sun et al., Chem. Mater. 12, 2617-2624 (2000), which are incorporated herein by reference.